1. Technical Field
The present invention relates to a hybrid-type superconducting fault current limiter, and more specifically, to an improvement in operational reliability of a hybrid-type superconducting current limiter which is capable of eliminating an arc current of a circuit breaker that might be occurring while a high-speed switch transfers to a current limiting unit the fault current detected by a superconductor when the fault current flows in the fault current limiter.
2. Discussion of the Related Art
A superconductor exhibits zero resistance during its normal operation state in a system, however, when a fault current flows in the system, the superconductor is quenched, and therefore, produces resistance which limits the fault current. At this time, the resistance may cause considerable energy to be applied to the current limiter. As a voltage applied to the system that runs the superconductor is high, the energy flowing in the superconductor correspondingly increases due to the impedance produced at the superconductor. Accordingly, lots of superconductors are needed to distribute the energy.
However, the superconductor is expensive in price and lots of superconductors mean large volume, which may increase the costs for installing and cooling the superconductor. To overcome the above problems, a hybrid-type superconducting fault current limiter including an existing circuit breaking means and a small number of superconductors has been disclosed (See Korean Patent Application No. 10-2006-0077520 filed on Aug. 17, 2006).
FIG. 1 is a circuit diagram illustrating a structure of a hybrid-type superconducting fault current limiter according to a prior art.
Referring to FIG. 1, the fault current limiter includes a main circuit and an auxiliary circuit. The main circuit includes a high-speed switch 2, which has a circuit breaker 2a, a driving coil 2b, and an electromagnetic repeller 2c, and a short contact 2d, and a superconductor 1 connected in series with the circuit breaker 2a. The auxiliary circuit includes a current limiting unit 3 for limiting the fault current. The circuit breaker 2a is mechanically linked with the electromagnetic repeller 2c and the short contact 2d, so that, in a case where a current is applied to the driving coil 2b, an eddy current is developed across the electromagnetic repeller 2c, and therefore, the circuit breaker 2a is operated together with the short contact 2d. The current limiting unit 3 may include a power fuse, a resistor, a reactor, a superconductor, a semiconductor element, and the like which have impendence to limit the fault current.
FIG. 2 is a graph showing a test result of the hybrid-type superconducting fault current limiter, and FIG. 3 is a graph showing operational points of time of the hybrid-type superconducting fault current limiter based on the test result of FIG. 2. In FIGS. 2 and 3, the current limiting unit 3 includes a current limiting fuse and a resistor connected in parallel with each other.
Referring to FIGS. 1 to 3, an electric current Itot passes through the closed circuit breaker 2a and superconductor 1 during a normal operation state without any fault, so that loss caused by occurrence of resistance is substantially ‘0’. In a case where a fault current flows into the fault current limiter, however, the superconductor 1 starts to be quenched at very high speed (6-1), and impedance developed at the superconductor 1 bypasses the fault current to the driving coil 2b. Since the fault current limiter is designed such that the impedance is very small, a low voltage alone is instantly applied from the electric power system to the fault current limiter, so that a small number of superconductors may be sufficient to implement the fault current limiter. At this time, a magnetic field is generated by the current flowing into the driving coil 2b, and an eddy current having a diamagnetic component is induced at the repeller 2c located over the driving coil 2b. Accordingly, the repeller 2c moves fast and opens the circuit breaker 2a that is mechanically linked with the repeller 2c thereby to cut off the inflow of the fault current into the superconductor 1 (6-2). The minute that the circuit breaker 2a is open, an arc current occurs across the circuit breaker 2a, which causes the fault current to continue to flow into the superconductor 1. To eliminate the arc current, the fault current limiter is designed to close the short contact 2d that is mechanically linked with the electromagnetic repeller 2c (6-3). The short contact 2d serves to remove the arc current occurring across the circuit breaker 2a connected in series with the superconductor 1, and protect the driving coil 2b from inflow of the fault current. The whole fault current is transferred through the short contacts 2d to the auxiliary circuit, and therefore, the arc current across the circuit breaker 2a is eliminated (64) and then the fault current is transferred to the auxiliary circuit and reduced by the current limiting unit 3 (6-5). Here, the current limiting unit 3 is designed to lag behind the superconductor 1 and high-speed switch 2 in operation.
FIG. 4 is a graph showing a test result when the electric arc was not blocked by the main circuit of the fault current limiter, and FIG. 5 is a graph showing a test result of Imain when the electric arc was not blocked by the main circuit of the fault current limiter. Referring to FIGS. 4 and 5, the electric arc occurring across the circuit breaker 2a that is connected in series with the superconductor 1 could not be sufficiently removed before the current limiting unit 3 starts to operate due to difference in impedance between the main circuit and the auxiliary circuit that functions to limit the current (7-1). Accordingly, an electric arc is reproduced across the circuit breaker 2a due to the difference in impedance between the main circuit and the auxiliary circuit (7-2), which can reduce the arc impedance, so that the fault current can go through the superconductor 1 that changed into a normal conductive state and the circuit breaker 2a that becomes conductive due to the electric arc. At this time, most of voltage is applied to the superconductor 1 that is in a normal conductive state, so that the fault energy may flow into the superconductor 1, thus damaging the superconductor 1.